Fantasy Sports (and particularly Fantasy Football) are heavily played because they are easy to play online and the scoring systems mostly make sense to the participants and rewards their smart decisions. There is both a realized value (ad revenue sales) and an inherent beneficial value to society (social interaction between people of all ages in vast numbers that help to unite our society by underscoring those interest that we share).
Annually NCAA Basketball tournament brackets are filled out in huge numbers because they can be done online or on a paper bracket (ease of use) and the odds are long enough to level the playing field between the sport expert and the kid who has never seen a game before. The NCAA tournament bracket craze offers the same benefits.